Water Ice Deposits Research Articles

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

The Mini‐RF radar instrument on the Lunar Reconnaissance Orbiter spacecraft mapped both lunar poles in two different RF wavelengths (complete mapping at 12.6 cm S‐band and partial mapping at 4.2 cm X‐band) in two look directions, removing much of the ambiguity of previous Earth‐ and spacecraft‐based radar mapping of the Moon's polar regions. The poles are typical highland terrain, showing expected values of radar cross section (albedo) and circular polarization ratio (CPR). Most fresh craters display high values of CPR in and outside the crater rim; the pattern of these CPR distributions is consistent with high levels of wavelength‐scale surface roughness associated with the presence of block fields, impact melt flows, and fallback breccia. A different class of polar crater exhibits high CPR only in their interiors, interiors that are both permanently dark and very cold (less than 100 K). Application of scattering models developed previously suggests that these anomalously high‐CPR deposits exhibit behavior consistent with the presence of water ice. If this interpretation is correct, then both poles may contain several hundred million tons of water in the form of relatively “clean” ice, all within the upper couple of meters of the lunar surface. The existence of significant water ice deposits enables both long‐term human habitation of the Moon and the creation of a permanent cislunar space transportation system based upon the harvest and use of lunar propellant.

Thermal Stability of Volatiles in the North Polar Region of Mercury

Thermal models for the north polar region of Mercury, calculated from topographic measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. MESSENGER measurements of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface. We propose that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived organic material.

Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements

Measurements by the Neutron Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft show decreases in the flux of epithermal and fast neutrons from Mercury's north polar region that are consistent with the presence of water ice in permanently shadowed regions. The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 30 cm thick that is less rich in hydrogen. Combined neutron and radar data are best matched if the buried layer consists of nearly pure water ice. The upper layer contains less than 25 weight % water-equivalent hydrogen. The total mass of water at Mercury's poles is inferred to be 2 × 10(16) to 10(18) grams and is consistent with delivery by comets or volatile-rich asteroids.

Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles

Measurements of surface reflectance of permanently shadowed areas near Mercury's north pole reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice, whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation. Impacts of comets or volatile-rich asteroids could have provided both dark and bright deposits.

Measurement of the accumulation of water ice on optical components in cryogenic vacuum environments

Standard vacuum practices mitigate the presence of water vapor and contamination inside cryogenic vacuum chambers. However, anomalies can occur in the facility that can cause the accumulation of amorphous water ice on optics and test articles. Under certain conditions, the amorphous ice on optical components shatters, which leads to a reduction in signal or failure of the component. An experiment was performed to study and measure the deposition of water (H2O) ice on optical surfaces under high-vacuum cryogenic conditions. Water was introduced into a cryogenic vacuum chamber, via a hydrated molecular sieve zeolite, through an effusion cell and impinged upon a quartz-crystal microbalance (QCM) and first-surface gold-plated mirror. A laser and photodiode setup, external to the vacuum chamber, monitored the multiple-beam interference reflectance of the ice-mirror configuration while the QCM measured the mass deposition. Data indicates that water ice, under these conditions, accumulates as a thin film on optical surfaces to thicknesses over 45 microns and can be detected and measured by nonintrusive optical methods which are based upon multiple-beam interference phenomena. The QCM validated the interference measurements. This experiment established proof-of-concept for a miniature system for monitoring ice accumulation within the chamber.

The m‐chi decomposition of hybrid dual‐polarimetric radar data with application to lunar craters

We introduce a new technique derived from the classical Stokes parameters for analysis of polarimetric radar astronomical data. This decomposition is based on m (the degree of polarization) and chi (the Poincaré ellipticity parameter). Analysis of the crater Byrgius A demonstrates how m‐chi can more easily differentiate materials within ejecta deposits and their relative thicknesses. We use Goldschmidt crater to demonstrate how m‐chi can differentiate coherent deposits of water ice. Goldschmidt crater floor is found to be consistent with single bounce Bragg scattering suggesting the absence of water ice and further corroborating adsorbed H to mineral grains or an H2O frost as plausible explanations for a H2O/OH detection by near‐infrared instruments.

Climate-driven deposition of water ice and the formation of mounds in craters in Mars’ north polar region

This paper explores the origins and evolution of ice-rich interior mounds found within craters of the north polar region of Mars. We present a systematic study of impact craters above 65°N, and identify 18 craters that have interior mounds. At least 11 of these mounds are composed of water ice and geometric similarities suggest that dune-covered mounds may also have a water ice core. The mounds are found in the deeper craters in the north polar area and we suggest that these form a specific microclimate favorable for mound initiation and growth. It is likely that at least seven of the mounds have evolved as individual outliers, rather than conterminous with the main polar cap. Our observations suggest that the mounds are built up by atmospheric deposition, similar to that of the north polar layered deposits. Using a combination of remote sensing techniques enabling topographic, spectral, radar and image data analyses, we have documented the morphology, composition and stratigraphy of selected mounds. We advance and test four hypotheses for formation of these mounds: artesian outpouring from a deep aquifer, hydrothermal activation of ground ice, remnants of a more extensive polar cap, and atmospheric deposition on ice caps in meteorologically isolated locations. We propose that during periods when the perihelion was located in northern summer (most recently 10–25ka before present) the microclimate in these craters retarded the sublimation of CO2 and water ice in northern spring, thus creating a cold trap for volatiles released as the seasonal cap retreated. This created a thick enough deposit of water ice to withstand sublimation over the summer and initiate a positive feedback leading to mound-building. Mounds without complete dune-cover may be in dynamic equilibrium with the ambient climate and show evidence of both present-day and past periods of erosion and aggradation. We conclude that the water ice mounds formed in deep impact craters in Mars’ north polar region may contain sensitive records of past polar climate that may enhance our understanding of the CO2–H2O system in the polar regions.

Areas of permanent shadow in Mercury's south polar region ascertained by MESSENGER orbital imaging

Radar‐bright features near Mercury's poles have been postulated to be deposits of water ice trapped in cold, permanently shadowed interiors of impact craters. From its orbit about Mercury, MESSENGER repeatedly imaged the planet's south polar region over one Mercury solar day, providing a complete view of the terrain near the south pole and enabling the identification of areas of permanent shadow larger in horizontal extent than approximately 4 km. In Mercury's south polar region, all radar‐bright features correspond to areas of permanent shadow. Application of previous thermal models suggests that the radar‐bright deposits in Mercury's south polar cold traps are in locations consistent with a composition dominated by water ice provided that some manner of insulation, such as a thin layer of regolith, covers many of the deposits.

Pressure dependent trace gas trapping in amorphous water ice at 77 K: Implications for determining conditions of comet formation

The nature of cometary volatile materials is subject to debate. Theoretical models of cometary nuclei and laboratory studies suggest that these objects could be made of amorphous water ice in addition to other volatile molecules and refractory grains. This water ice structure has the ability to encapsulate the gases of surrounding environment, reflecting the physical and chemical conditions during their deposition. Therefore, the knowledge of the chemical composition of volatile molecules trapped in amorphous water ice provides a tool for probing the formation environment of cometary ice grains. Experimental studies of gas trapping efficiency in amorphous water ice have been previously conducted mostly under kinetic conditions, where dynamic pumping and temperature gradients prevented rigorous calibrations. In this work, we investigated the trapping efficiencies of Ar, CO, CH4, Kr and N2 by depositing water vapor as ice in the presence of trace gases in a volume submerged in liquid nitrogen at 77K. The gas trapping efficiencies were determined simply by monitoring the pressure difference of the trace gases before and after the deposition of a known amount of water molecules as amorphous ice.Our results show that the trapped gas to water molecule ratio in amorphous ice is controlled primarily by the partial pressure of the gas during water ice deposition, and is independent of the ice deposition rate as well as the gas to water ratio in the vapor phase. The trapping efficiencies of gases decrease in the order of Kr>CH4>CO>Ar>N2 in accordance with previous studies. Assuming that the water ice structure of comets is at least partially amorphous water ice at the time of their formation, these results suggest that the total pressure and composition of the surrounding environment of amorphous ice formation are significant controlling factors of trace gas concentrations in cometary ice. This further indicates that the evolution of the solar nebula and timing of cometary ice condensation can also be important parameters in linking the volatile contents of comets and their formation process.

基于"嫦娥一号"激光测高数据的月球极区光照条件研究

The illumination condition of the lunar polar regions can be used to analyze the choice of ideal landing sites and to conduct the investigation of possible water ice deposits. The laser altimeter data obtained by 'Chang/E-1' can provide high accuracy digital elevation models (DEM) for polar illumination model. In this study, we establish the mathematical model to study the polar illumination condition, by combining 'Chang' E-1' altimeter data and known parameters related to lunar orbit. The calculation period of illumination rate is 19 years, which is longer than the node precession period (18. 6 years). In this paper, the horizon method is used instead of the ray-tracing method, and the results show: (1) We don't find the areas that receive permanent illumination; (2) There are regions that are in constant darkness; (3) The edge of the craters can be found to be as continuous illumination region in the north summer or the south summer; (4) The Shackleton crater is one of the main targets as landing sites of the lunar rover.

Mars Odyssey neutron data: 2. Search for buried excess water ice deposits at nonpolar latitudes on Mars

[1] Global maps of hydrogen abundance near the surface of Mars, interpreted as the mass percent of water-equivalent hydrogen (WEH) have been generated from measurements of neutron leakage fluxes from Mars. Although these data provide an unambiguous indicator of the presence of WEH, quantitative details of its magnitude and burial depth depend on the model of the host regolith that is used to interpret the data. Previous models assumed a spatially uniform surface cover layer having a one-to-two mass percent of WEH and thickness D covering a semi-infinite ice-rich deposit. These assumptions allowed the derivation of the relative proportions of ice and regolith in the lower layer, which had been used to create global maps of WEH in the near surface. In this paper we develop a new method that determines, from the Mars Odyssey Neutron Spectrometer (MONS) data, a self-consistent model of the WEH content of both the upper and lower layers as well as the thickness (D) of the upper layer. The results of our model suggest that large areas at nonpolar latitudes may contain water ice deposits that have abundances that are larger than can be held by normal pore volumes. These deposits are buried less than about 1 m below the surface and may represent buried water ice or high concentrations of hydrous minerals. Intriguingly, the most definitive MONS evidence at intermediate latitudes for excess ground ice corresponds to a region of Arcadia Planitia within which High Resolution Imaging Science Experiment (HiRISE) has observed fresh icy craters.

Simulations of a comet impact on the Moon and associated ice deposition in polar cold traps

Modeling results of the water vapor plume produced by a comet impact on the Moon and of the resulting water ice deposits in the lunar cold traps are presented. The water vapor plume is simulated near the point of impact by the SOVA hydrocode and in the far field by the Direct Simulation Monte Carlo (DSMC) method using as input the SOVA hydrocode solution at a fixed hemispherical interface. The SOVA hydrocode models the physics of the impact event such as the surface deformation and material phase changes during the impact. The further transport and retention processes, including gravity, photodestruction processes, and variable surface temperature with local polar cold traps, are modeled by the DSMC method for months after impact. In order to follow the water from the near field of the impact to the full planetary induced atmosphere, the 3D parallel DSMC code used a collision limiting scheme and an unsteady multi-domain approach. 3D results for the 45° oblique impact of a 2 km in diameter comet on the surface of the Moon at 30 km/s are presented. Most of the cometary water is lost due to escape just after impact and only ∼3% of the cometary water is initially retained on the Moon. Early downrange focusing of the water vapor plume is observed but the later material that is moving more slowly takes on a more symmetric shape with time. Several locations for the point of impact were investigated and final retention rates of ∼0.1% of the comet mass were observed. Based on the surface area of the cold traps used in the present simulations, ∼1 mm of ice would have accumulated in the cold traps after such an impact. Estimates for the total mass of water accumulated in the polar cold traps over 1 byr are consistent with recent observations.

Models of high velocity impacts into dust-covered ice: Application to Martian northern lowlands

The detection of fresh impact craters with bright floors and ejecta (arising from fresh clean water ice) in the northern lowlands of Mars ( Byrne et al., 2009b, Science 325, 1674), together with observations of polygonal structures and evidence from the Phoenix probe, suggests that there are substantial water ice deposits just below the surface over large areas. Specifically in cases of the larger craters observed, the impacts themselves may have raised the temperature and the pressure of the water ice deposits locally to values which allow phase changes. In this paper, we use smoothed particle hydrodynamics to model hyper-velocity impacts. We estimate peak shock pressures in a solid water ice target covered by a layer of loose material, modeled by pre-damaged dunite. In addition, we account for the possibility of a thin layer of sub-surface water ice by using a three-layer model where the ice is surrounded by dunite. We find that the peak shock pressures reached in the simulated events are high enough to produce several 100–1000 kg of liquid water depending upon the impact parameters and the exact shock pressure needed for the phase change. A difficulty remains however in determining whether liquid is generated or whether a type of fluidized ice is produced (or indeed some combination of the two). We also note that the process can become rather complex as the number of layers increases because of reflections of the shock at sub-surface boundaries—a process which should lead to increased fluidization.

Initial results for the north pole of the Moon from Mini‐SAR, Chandrayaan‐1 mission

We present new polarimetric radar data for the surface of the north pole of the Moon acquired with the Mini‐SAR experiment onboard India's Chandrayaan‐1 spacecraft. Between mid‐February and mid‐April, 2009, Mini‐SAR mapped more than 95% of the areas polewards of 80° latitude at a resolution of 150 meters. The north polar region displays backscatter properties typical for the Moon, with circular polarization ratio (CPR) values in the range of 0.1–0.3, increasing to over 1.0 for young primary impact craters. These higher CPR values likely reflect surface roughness associated with these fresh features. In contrast, some craters in this region show elevated CPR in their interiors, but not exterior to their rims. Almost all of these features are in permanent sun shadow and correlate with proposed locations of polar ice modeled on the basis of Lunar Prospector neutron data. These relations are consistent with deposits of water ice in these craters.

Lunar Exploration Neutron Detector for the NASA Lunar Reconnaissance Orbiter

The design of the Lunar Exploration Neutron Detector (LEND) experiment is presented, which was optimized to address several of the primary measurement requirements of NASA’s Lunar Reconnaissance Orbiter (LRO): high spatial resolution hydrogen mapping of the Moon’s upper-most surface, identification of putative deposits of appreciable near-surface water ice in the Moon’s polar cold traps, and characterization of the human-relevant space radiation environment in lunar orbit. A comprehensive program of LEND instrument physical calibrations is discussed and the baseline scenario of LEND observations from the primary LRO lunar orbit is presented. LEND data products will be useful for determining the next stages of the emerging global lunar exploration program, and they will facilitate the study of the physics of hydrogen implantation and diffusion in the regolith, test the presence of water ice deposits in lunar cold polar traps, and investigate the role of neutrons within the radiation environment of the shallow lunar surface.

Characterization of the nighttime low‐latitude water ice deposits in the NASA Ames Mars General Circulation Model 2.1 under present‐day atmospheric conditions

This effort advances the exploration of the current Martian water cycle by analyzing the nighttime equatorial water ice deposits in the NASA Ames Mars General Circulation Model (version 2.1). The possibility that the current Martian water cycle plays a role in the generation or maintenance of the longitudinally confined tropical maxima of water‐equivalent hydrogen (WEH) in the near‐surface regolith is also investigated. Mars Odyssey Orbiter Gamma Ray suite observations indicate that tropical latitudes exhibit longitudinally distinct maxima of the presence of near‐surface hydrogen, likely in the form of hydrated minerals, in the Arabia and Tharsis regions. Atmospheric numerical simulations with the NASA Ames Mars General Circulation Model produce an annual water cycle that faithfully reproduces the cycle derived from Mars Global Surveyor Thermal Emission Spectrometer measurements. This simulated water cycle produces maxima of near‐surface tropical water condensation at longitudes consistent with the WEH maxima regions, though these maxima are shifted north of the Odyssey Gamma Ray suite WEH regions. While the simulated pattern of near‐surface tropical water condensation does exhibit a longitudinal structure consistent with the observed WEH maxima, it is concluded that the current water cycle is not playing a dominant role in the formation or retention of those maxima. Thus, atmospheric involvement in the formation of the WEH‐rich regions must occur during past climatic conditions, if at all.

Distribution and orientation of northern-hemisphere gullies on Mars from the evaluation of HRSC and MOC-NA data

Martian gullies show morphologic characteristics that indicate the presence of a flowing liquid during their formation process. However, the current climate in the northern hemisphere of Mars does not allow the existence of substantial amounts of liquid water and, therefore, an assessment of the regional distribution and characteristics of gullies is required in order to understand the most recent climatic development of Mars. We examined High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera—Narrow Angle (MOC-NA) data covering the northern hemisphere in order to determine the distribution, geological setting and orientation of gullied slopes. We used these results to test the consistency of formation models proposed in earlier works and to define the climate dependence. Gully orientations were analyzed only at impact crater walls, because craters have an azimuthally uniform distribution of slopes. This is ideal when investigating the influence of insolation on the distribution and development of landforms. We examined 230 HRSC and ~ 21,000 MOC-NA images and identified more than 2200 gullies on HRSC and more than 3100 gullies in MOC-NA data. Both surveys show the highest densities of gullies between 35°N and 55°N and a shift of gully orientation at 40°N from pole-facing slopes to slopes facing the equator. Orientation trends as well as the latitudinal distribution of identified gullies strongly support the view of gully formation caused by insolation/climatic conditions. This result suggests that formation mechanisms based on the assumption of atmospheric water-ice deposition proposed in earlier work are more likely than processes related to groundwater flow. Further support for a theory related to deposition is gained through observation of the high occurrence of gullies at isolated knobs and hills, which cannot be associated with formation models based upon groundwater flow.

The Mercury Gamma and Neutron Spectrometer (MGNS) on board the Planetary Orbiter of the BepiColombo mission

The main goals and objectives are presented for the Mercury Gamma and Neutron Spectrometer (MGNS) to be flown on board the Mercury Planetary Orbiter of ESAs BepiColombo mission. The instrument design is discussed and from an analysis of its properties, it is shown that the MGNS is capable of measuring the elemental composition of the shallow subsurface, providing a sensitive tool to discriminate between a number of proposed models for Mercury's origin. The instrument will also test for the presence of water ice deposits on the permanently shadowed polar craters of the planet.

Lack of Exposed Ice Inside Lunar South Pole Shackleton Crater

The inside of Shackleton Crater at the lunar south pole is permanently shadowed; it has been inferred to hold water-ice deposits. The Terrain Camera (TC), a 10-meter-resolution stereo camera onboard the Selenological and Engineering Explorer (SELENE) spacecraft, succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor, based on the crater shape model derived from the TC data, is less than approximately 90 kelvin, cold enough to hold water-ice. However, at the TC's spatial resolution, the derived albedo indicates that exposed relatively pure water-ice deposits are not on the crater floor. Water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all.

Martian high latitude permafrost depth and surface cover thermal inertia distributions

Martian high latitudes have thermal properties consistent with an extensive high thermal inertia permafrost layer near the surface. Surface cover thermal inertias and permafrost depths at Martian high latitudes (50°–80°N/S) are derived from Thermal Emission Spectrometer (TES) data and compared with previously published water ice depths determined from the Mars Odyssey Neutron Spectrometer (MONS). The depth to the permafrost layer is correlated with surface cover thermal inertia, albedo, and latitude in general agreement with predicted trends of water ice stability. Comparison of permafrost depths with water ice rich layer depths derived from MONS data displays good qualitative agreement, although a divergence is present at greater burial depths. This disparity may be due to the presence of hydrated minerals at shallow depths or a lower than expected permafrost thermal inertia corresponding with low water ice concentrations at greater depths. Surface cover thermal inertias are greater in the northern high latitudes than in the south and differences between these and previous results will have significant effects on the predicted depth of Martian water ice stability. Several regions in the northern hemisphere display high surface cover thermal inertia associated with possible receding water ice deposits that are shallow enough to influence diurnal surface temperatures. Significant lateral and vertical heterogeneity in water ice distributions are present and the Martian regolith is likely more complicated than can be described by simple two layered models and a single mode of water ice emplacement.